Cell wall and oxidative metabolisms of ripening ?Paluma? guava under potassium fertilization

Guava (Psidium guajava L.) is an important tropical fruit crop, and its nutritional status is directly associated with the quality and postharvest conservation of the fruits. Several agronomic events are involved with changes in the flavor and texture of the fruits, which are associated with the postharvest delay of fruit maturation, especially cell wall loosening. In this study, 'Paluma' guava trees were treated with three doses of potassium fertilization (50 [producer dose -control], 100, and 150 g K2O plant-1) during the phenological phase of fruit growth, and the fruits were evaluated in three maturity stages (green yellow, yellow greenish, and yellow). The completely randomized design and fruits from three plants were used as replicates per treatment. The fruits were evaluated by the changes in pectin content and cell wall enzyme activity (pectin methylesterase and polygalacturonase) and oxidative metabolism enzymes (polyphenoloxidase and peroxidase activity). The results demonstrate that fruits from plants fertilized with 100 g K2O plant-1 were firmer (59.24 N), in which the yellow-greenish stage presented higher total (3.69%) and insoluble pectin (2.58%) contents. Furthermore, fruits at yellow-greenish stage presented lower pectin methylesterase (370.9 mU g-1 FW) and polygalacturonase (110.1 mU g-1 FW) cell wall enzyme activity, reflecting the better resistance of the cell wall and possibly longer postharvest life. As a practical application, the use of 100 g of K2O plant-1 during production and harvesting of "Paluma" guava at the yellow-greenish stage can maintain fruit firmness, improve quality, and reduce postharvest losses.